1. Field of the Invention
This invention is related to the field of electro-optical systems in general and to the use of liquid crystals in electro-optical systems in particular.
2. Description of Prior Art
The utilization of nematic liquid crystals in the fabrication of electro-optical devices has been well established. For certain applications, it is desirable to orientate the directors of these crystals such that they assume a tilted alignment with respect to the surface of a substrate. Such an alignment is necessary for the television projection display and color symbology light values made by Hughes Aircraft Company.
In general, a tilted alignment is desirable if, an electric field is applied across the liquid crystal normal to the surfaces, and if the liquid crystal has a negative dielectric anisotropy so that the molecules are tilted towards a parallel position.
Prior art processes for aligning liquid crystals involved the use of either soluble ionic alignment agents or polymeric surface coatings.
Ionic aligning agents included tetralkylammonium salts (U.S. Pat. No. 3,656,834 by I. Haller et al. dated Apr. 18, 1972), barium stearate (I. Haller, Appl. Phys. Lett. 24, 349[1974]), or lecithin (F. K. Kahn et al., Proc. IEEE 61, 823[1973]).
The use of polymeric silicone coatings to align liquid crystals has been reported by Kahn, et al., lic. cit. as has the use of a plasma-polymerized film (J. C. Dubois et al. Appl. Phys. Lett. 24, 297[1974]).
A soluble polyamide has also been used to align liquid crystals (W. Haas, et al., Phys. Rev. Lett. 25, 1326[1970]).
Each of the alignment process disclosed above has been shown to yield orientated liquid crystals with some significant limitations. In the case of ionic alignment agents, the undesirable characteristic is the conductivity of the resulting mixture. Devices fabricated from liquid crystals orientated via a polymeric aligning agent exhibit slow response times and poor alignment stability.